THE MOLECULAR BASIS OF MOSQUITO CONTROL BY BACTERIA

Bacillus thuringiensis has been in commercial use as a microbial
insecticide for many years. Until recently, it had been assumed that this
crystal-forming bacterium was only entomocidal for the agriculturally
important Lepidoptera insects. Now, however, a new variety, Bacillus
thuringiensis var. israelensis, has been found which is effective against
both mosquitoes and black flies. This unexpected observation poses the
question how var. israelensis differs from the other B. thruingiensis
strains and whether these differences can be further exploited.
Information pertaining to these questions should be relevant to an
understanding of the molecular basis of host specificity in the microbial
insecticides. Since the protein crystal is responsible for the entomocidal
activity exhibited by B. thuringiensis, it is the logical starting point
for a study on the altered host specificity of var. israelensis.
Accordingly, we will compare purified crystals from var. israelensis and
selected Lepidoptera-active strains with regard to as many relevant
biochemical parameters as possible. Specifically, we will examine: 1/ The
influence of crystal morphology (e.g. Amorphous vs. Discrete) on toxicity.
2/ The conditions necessary to solubilize var. israelensis crystals.
Solubility is a prerequisite for toxicity. 3/ The presence of chitin
barrier in the mosquito larval gut lining. Does var. israelensis have
chitinase activity? 4/ The presence and function of interchain disulfide
bonds in the var. israelensis crystal. These unusual bonds are the
dominant structural features of other B. thuringiensis crystal types. and
5/ The chemical identity of the crystal digestion product which acts as the
ultimate mosquito toxin. The purification and characterization of the var.
israelensis crystal is underway in our laboratory. Additionally, we have
recently demonstrated that the crystal's larval toxicity can be simulated
by cyclic peptide ionophores.